Dutch police were holding an Austrian man after the discovery of a father and his adult children who were believed to have stayed hidden in a remote farmhouse for years, officials said Wednesday.

The mystery surrounding the case in the village of Ruinerwold in the northern province of Drenthe also deepened with reports that one of the children had been active on social media this year.

Police said they discovered a father and five children aged between 18 and 25 on Monday and arrested a 58-year-old man -- not the father -- for failing to cooperate. They initially spoke of six children but later revised the number down.

The Austrian foreign ministry confirmed that a Austrian national from Vienna was being held in relation to the case, but said the man did not want to have contact with Austrian officials.

Ministry spokesman Peter Guschelbauer told AFP said they did not know the exact grounds for his arrest. Dutch authorities said all others in the cellar case were Dutch, Guschelbauer added.

Dutch and Austrian media said the man was known as "Josef, the Austrian" and the one holding the group in the cellar, renting the house and planting vegetables for them in the garden.

According to media reports, the family had spent years "waiting for the end of time", with Ruinerwold's mayor telling journalists "I have never come across anything like this before."

The oldest of the children, a 25-year-old who has only gone by the name of "Jan", kept a Facebook account and who for the first time in nine years posted an update in June, according to Dutch media.

"Started a new job at Creconat," popular daily tabloid De Telegraaf quoted him as saying.

The company is affiliated to another in the nearby town of Meppel and was raided by police on Monday, the paper said, adding it belonged to the 58-year-old Austrian man.

"Jan" also posted pictures of himself "surrounded by trees" and in and near Ruinerwold, as well as links to products and rallies including the recent climate marches, the paper said.

On his site on the jobs site LinkedIn, meanwhile, he wrote that his parents had run a successful business until his mother died in 2004.

Local news station RTV Drenthe, which first reported the story, said police were alerted after "Jan" walked into a village bar on Sunday evening.

The dishevelled man, unwashed and wearing old clothes said "he has not been 'outside' for the past nine years," bar owner Chris Westerbeek told RTV Drenthe.

Upon investigation, police discovered a hidden staircase behind a cupboard leading to a cellar where the father and his children were hiding.